A vehicular storage battery provides DC power for engine start-up and the like. The vehicle has an alternator which is mechanically driven by the operating engine to provide current for charging the battery. The alternator includes a diode network for converting the AC to DC. The alternator is usually three phase in nature, so that the diode network includes three pairs of diodes, each pair providing the DC for approximately one third of the cycle, with a ripple therein. So long as the diodes are functioning properly, the amount of ripple normally encountered in the alternator output waveform is rather small. If one or more of the charging diodes should fail, such as due to vibration, shock, electrical burn out or the like, the alternator output waveform becomes considerably distorted and the ripple amplitude increases since for a substantial portion of each output waveform period, no rectification takes place.
Servicemen use an alternator tester to evaluate the alternator in order to determine whether it is operating properly. Such apparatus commonly has an ammeter to display the amount of average current being delivered by the alternator. A malfunction is indicated when the average current does not meet specifications. One possible cause is one or more defective diodes. As mentioned above, the ripple supplied by a properly operating alternator is small, but increases markedly if one or more of the diodes is defective. Presently available alternator testers have an indicator which displays information as to such ripple amplitude. If the ripple amplitude is excessive, the serviceman would disassemble the alternator and individually evaluate each diode.
Vehicular alternators vary in the amount of current they can deliver, from say 25 amps to 100 amps or more. Ripple of a given amplitude supplied by a high-current alternator may be perfectly acceptable but if present to the same amplitude on a low-current alternator would indicate a defect.
Actually, most alternator testers give a "good/bad" indication and the serviceman does not know the amplitude of the ripple. The tester is calibrated so that if the ripple has an amplitude greater than a certain value then it is bad and if less than such value it is good. Such a tester can be used to evaluate an alternator which has a given current output. An alternator having a different current capability would require a different modified tester.